1. Field of the Invention.
This invention relates to a ringing choke converter power supply device (herein abbreviated "RCC power supply") which is classified as a DC-DC converter; and more particularly, to an improvement therein for stabilizing the output voltage thereof.
2. Description of the Prior Art.
Known RCC power supplies are disclosed, for example, in Japanese UM Kokai 62-98490 and 62-111788. FIG. 1 is a block diagram depicting a prior art RCC power supply, wherein an NPN transistor is used as a switching element. In this case, an input terminal corresponds to a collector terminal, an output terminal corresponds to an emitter terminal, and a control terminal corresponds to a base terminal. In other types of switching elements, for example, a PNP transistor, the input terminal corresponds to the emitter terminal, the output terminal corresponds to the collector terminal, and the control terminal corresponds to the base terminal The situation is the same for an FET.
In FIG. 1, a transformer T includes a primary winding N.sub.p, a voltage detection winding N.sub.B, and a secondary winding N.sub.s which are all marked with (*) to indicate the anode thereof. In a main transistor Q, which is used for switching, a cathode of primary winding N.sub.p is connected to the collector terminal, while the control input terminal is connected to the base terminal. A starting resistance R.sub.g is interposed between an anode terminal of primary winding N.sub.p and the base terminal of main transistor Q.
A feedback diode D2 and a Zener diode Dz serve as an indirect feedback path to provide a constant voltage circuit. A connective state will now be described. The anode terminal of voltage detection winding N.sub.B is connected to feedback diode D2 to which Zener diode Dz is connected in series. A cathode of feedback diode D2 is connected via a current limit resistance Rb to the base terminal of main transistor Q and also to an anode of voltage detection winding N.sub.B. The cathode of Zener diode Dz is connected to the base terminal of main transistor Q. A capacitor C1 has one end connected to a connecting point between feedback diode D2 and Zener diode Dz; and the other end connected to a voltage reference line. Zener diode Dz acts as a constant voltage. power supply and also behaves as an error signal generation circuit of an output voltage with respect to a reference voltage.
It is to be noted that the indirect feedback implies not a step of directly feeding back secondary outputs to stabilize the secondary output voltage, but, is instead, a step of indirectly feeding back the secondary output by using voltage detection winding N.sub.B. The indirect feedback is easier to accommodate primary and secondary insulations as compared to the direct feedback. Indirect feedback exhibits characteristics in which the construction is simple and provided at low cost. The secondary winding N.sub.s is rectification smoothed through diode D1 and capacitor C1, and is supplied in the form of an output voltage Vout to a load.
The description will next deal with the thus constructed device. Upon impression of an input voltage Vin, main transistor Q is fed with a base current via starting resistance R.sub.g, and is then brought into a switching operation mode. When main transistor Q is turned OFF, a rectifier diode D1 on the secondary side becomes conductive, and at the same moment feedback diode D2 connected to voltage detection winding N.sub.B becomes conductive. At this time, an output voltage Vns of secondary winding N.sub.s is substantially equalized to an output voltage Vout. On the other hand, a voltage Vnb of voltage detection winding N.sub.B is proportional to voltage Vns. Hence, a voltage Vc of capacitor C1 in which voltage Vnb is rectified, is eventually proportional to voltage Vout.
Voltage Vc rises with an increase in output voltage Vout. However, Zener diode Dz becomes conductive to cut the base current of main transistor Q, thereby speeding up the turning OFF of main transistor Q. The output voltage Vout is thus made constant.
There are various deficiencies and disadvantages in the prior art devices. For example, strictly speaking, voltage Vnb is not proportional to voltage Vns due to influence of leakage inductance. Also, no correction is made for drop in voltage which is associated with a DC resistance component of the output circuit and the forward electric potential V.sub.F of the rectifier diode
For these reasons, the indirect feedback system is inferior to the direct feedback system in terms of stability of output voltage. In the direct feedback system, an error signal is detected by comparing the output voltage Vout with a direct reference voltage Vref, and control is effected so that the error signal becomes zero.